Formed articles of inorganic fibers typified by ceramic fibers have been used for applications exposed to high temperatures, for example, industrial heat insulators, refractory materials, and packing materials. In recent years, such formed articles have also been used as cushioning materials (catalyst-holding material) for automotive exhaust gas cleaning devices. That is, such formed articles have also been used as mats for exhaust gas cleaning, in which mats are wound around catalyst-supporting members and arranged between catalyst-supporting members and metal casings when catalyst-supporting members are accommodated in metal casings.
For example, in the case where such an inorganic fiber formed article is processed as a heat insulator or processed into an automotive catalyst-holding material (mat), needling treatment (needle punching treatment) is commonly performed in order to control the thickness and the surface density. The mat thus treated by needling have needle marks.
Patent Literatures 1 and 2 disclose that the number of needle marks per unit surface area (needle mark density) of an inorganic fiber-formed article is 100 to 5000 marks/100 cm2 (1 to 5 marks/cm2). Patent Literature 3 discloses that the number of needle marks per unit surface area is 2 to 20 marks/cm2. Patent Literature 4 discloses that the needle mark density is preferably 50 to 250 marks/cm2.
In general, needle marks are substantially uniformly distributed over the entire surface of the inorganic fiber-formed article. However, FIG. 1 in Patent Literature 4 illustrates an inorganic fiber-formed article in which needle mark rows each including needle marks that lie in a line are arranged in parallel.
Patent Literature 1: JP2007-127112A
Patent Literature 2: JP2007-268514A
Patent Literature 3: JP2008-201125A
Patent Literature 4: JP2011-99182A
As described above, the thickness and surface density of the inorganic fiber-formed article are controlled by performing needling treatment on the inorganic fiber-formed article. The resilience and delamination strength of the inorganic fiber-formed article are also controlled by performing needling treatment.
Inorganic fiber-formed articles used as holding members or the like in a catalytic converter desirably have high delamination strength and also high resilience (surface pressure). However, in the inorganic fiber-formed article subjected to needling treatment, the delamination strength tends to increase and the surface pressure tends to decrease as the needle mark density increases. On the other hand, the surface pressure tends to increase and the delamination strength tends to decrease as the needle mark density decreases.